1. Field of the Invention
The present invention relates to a theft prevention apparatus for vehicles. More particularly, the present invention relates to an apparatus which produces an alarm sound in response to the detection of movements of the vehicle which may be caused when a person other than the driver attempts to steal the vehicle while it is parked.
2. Description of the Prior Art
There have been known theft prevention apparatus which can produce an alarm sound upon detection of unusual movements of a vehicle while it is parked. FIG. 4 of the accompanying drawings shows a conventional movement detector used in one such theft prevention apparatus.
The movement detector comprises a base plate 43 and a movable plate 44 which are spaced from each other and have confronting contacts 41, 42 provided thereon. When the movable plate 44 is moved up and down due to the action of a weight 45 attached to a distal end of the movable plate 44, the contacts 41, 42 are momentarily closed, whereupon the movement can be detected by the frequency of the closed states of the contacts 41, 42. An adjustment screw 46, when turned, changes the distance between the contacts 41, 42 so as to adjust the sensitivity of movement detection.
One of the problems with such prior movement detector is that sensitivity adjustment is difficult to perform with the screw 46, that is, it is difficult to positively determine how wide the distance between the contacts should be for the detection of attempts to steal the vehicle. Further, the distance between the contacts, even if correctly set initially, tends to vary due to vibrations as the vehicle is running. Accordingly, the prior movement detector does not provide stable and reliable operation.